The ATLAS Collaboration released its first result from Pb+Pb collisions at the LHC, and it’s a blockbuster. Their new paper, titled Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at √sNN = 2.76 TeV with the ATLAS Detector at the LHC, has been fast-tracked for acceptance in Physical Review Letters. In this paper ATLAS reports the observation of strong quenching in highly energetic jets (> 100 GeV) produced in Pb+Pb collisions at the LHC. The quenching is sufficiently strong that the trigger jet may properly be referred to as a monojet.


The effect is clearly seen in Figure 1 of their paper, reproduced above. The high  pT towers that dominate the Lego plot and the event display are unbalanced by a partner jet on the away side in azimuth. Instead, their appears to be enhanced low p emission over nearly the entire away side azimuthal distribution, mostly clearly seen in the towers of the beams-eye view event display.

A more quantitative analysis is of course provided in the paper, in which the evolution of the quenching is presented as a function of centrality, and also compared to HIJING events supplemented with unquenched jets from PYTHIA. For peripheral events, both the the energy asymmetry of the away-side jet and the distribution in azimuthal angle is in good agreement with those seen in p+p events and in the HIJING+PYTHIA Monte Carlo events. This is in sharp distinction to the most central Pb+Pb events, where the jet events have a pronounced asymmetry in energy and a significantly broadened angular distribution.

Of course, the disappearance of the away-side “jet” was discovered by the STAR collaboration in data collected during the first full energy RHIC run. But several aspects of the ATLAS result are even more striking: The observed quenching is for true jets, fully reconstructed in heavy ion events with complete and essentially hermetic hadronic calorimetry; the quenching remains to jet energies of (at least) 100 GeV; and the modification of the jet azimuthal energy distribution is clearly observed in the away-size distribution from the trigger jet. These are all firsts in heavy ion physics, and to see them in a Physical Review Letter a few days after the start of heavy ion physics running at the LHC is indeed remarkable.

The ATLAS results is featured in a CERN press release, dated today (26-Nov-10), where it is stated a similar result from CMS will follow shortly. Also posted today is a Symmetry Magazine article on the ATLAS result containing comments by Brian Cole and Peter Steinberg.

What A Difference a Day Makes (*)

The large value of charged multiplicity  dNch/dη = 1584 observed by ALICE in the 5% most central collisions, and the large value of elliptic flow they report in minimum bias collisions both suggest that the demise of the strong coupling paradigm at the LHC have been greatly exaggerated. I attribute this in part to “the tyranny of asymptotic freedom”, that is, the beauty of the running coupling constant in QCD has led to some wishful thinking about how fast a logarithmic term can vary. There is also the enormous appeal and simplicity of the “classical QGP” described as a nearly free, massless Boltzmann gas. But the data from RHIC, and now these first ever so exciting results from LHC, have taught us that there is also a wealth of fundamental physics and new phenomena in the strongly-coupled regime that is so much more interesting than a non-interacting gas!Predictions for dN_ch/d\eta excluded by ALICE

At the same time there has indeed been a demise, in fact several, in this case of the various predictions of (mostly) lower multiplicity densities than the value of ~1600 reported by ALICE. I was one of those who would have guessed the lower values of 1200-1300, but since that was only a guess, the surprise for me was a pleasant one. Ironically, earlier on the day this value was announced in Andrea Dainese’s talk at the LHCC , I had read with great interest this paper Hadron production at the LHC: Any indication of new phenomena by Levin and Rezaeian , describing in a very accessible fashion their predictions for LHC multiplicities based on saturation physics. There is also a nice discussion of the extensions incorporated in their approach over the KLN model, so it is more than a little puzzling that the simpler KLN model (at least in one of its various instantiations) better describes the Pb+Pb data at the LHC.

(See also Berndt Mueller’s post for a more quantitative discussion of what we learn from the first two ALICE papers on Pb+Pb collisions at the LHC.)

(*) The title of this post is drawn from one of the loveliest songs in the American jazz idiom.

First Pb+Pb Collisions at 2.76 TeV !

The long-awaited day has arrived, in spectacular fashion. The LHC has declared “Stable beams with ions”, and ALICE, ATLAS and CMS are all seeing events, and sharing their beautiful event displays with the world.

It is enormously satisfying to see the field of relativistic heavy ion physics take this tremendous (large) step on the energy frontier. Congratulations and kudos to all who have worked for decades to make this happen. Surely extraordinary discoveries await.

Update: There is now a press release from CERN with useful links to various event displays.

First LHC Data!

Noted with great pleasure- the first preprint on LHC data has been posted by the ALICE Collaboration.


This is a marvelous tribute to the hundreds of physicists, students, engineers and technicians, many of whom have invested nearly two decades towards this achievement.

The paper presents ALICE’s measurement of the charged particle pseudo-rapidity density and its distribution for |η|<1 in √s=900 GeV p+p collisions. The results, obtained from only 284 events,   are in good agreement with UA5 data for proton-antiproton collisions at this energy. But what’s important is this demonstration that ALICE can (very!) quickly extract physics data from a few collisions, suggesting that new results from new energy regimes will appear very quickly as the LHC increases the collision energy.

2008 Nobel Prize in Physics

The 2008 Nobel Prize in Physics was awarded, in half, to Yoichiro Nambu for his groundbreaking work on the mechanism responsible for the spontaneous breaking of symmetries in elementary particle physics. Nambu’s work, published in 1961 in two seminal articles co-authored by Giovanni Jona-Lasinio:

showed that a strong interaction among (nearly) massless fermions will lead the formation of a condensate of fermion-antifermion pairs in the vacuum, which breaks the symmetry associated with massless fermions, i.e. chiral symmetry, endowes the fermions with a dynamical mass, and leads to the emergence of massless bosons called Goldstone bosons. When applied to the isospin doublet of up- and down-quarks, the NJL mechanism explains the relative lightness and other properties of the pions, π+, π0, and π, as a consequence of their Goldstone boson character.

Nambu’s mechanism today is understood to be realized in the fundamental interaction of the strong nuclear interaction, quantum chromodynamics (QCD). Because of the analytical intractability of QCD, the NJL model is still used extensively an an effective model of chiral symmetry breaking in QCD. The central goal of relativistic heavy ion collisions is to heat the QCD vacuum to such a high temperature that the interaction among quarks is weakened and the broken symmetry is restored.

It has been recognized in recent years that chiral symmetry breaking is intimately linked to the other fundamental property of the QCD vacuum, quark confinement. Naïvely, the restoration of chiral symmetry and quark-deconfinement could occur at different temperatures. Numerical simulations of lattice QCD have shown that they occur together. An extension of the NJL model to include symmetry aspects of the color force, the PNJL model

provides an explanation of the mechanism responsible for the linkage of the two phenomena and thus for the existence of a single critical temperature threshold for the formation of a quark-gluon plasma exhibiting the full symmetries of QCD.

The Quest for the Ideal Fluid: Competition from Li-Atoms

In April of 2005 Brookhaven National Laboratory announced that scientists at RHIC had created the most ideal liquid ever observed in nature. Now, 3 years later, RHIC is facing competition to this claim from strongly interacting ultra-cold Fermi gases. (more…)

The One That Started It All

The one that started it all was the first experimental paper from RHIC. (more…)

Heavy Quark Primer

The use of charm quarks produced in ultra-relativistic heavy-ion collisions as QGP probes has a rich tradition, starting with the seminal paper by Matsui and Satz, predicting the suppression of J/Psi production in a QGP: (more…)

Hydro Primer

This post provides a list on historically relevant work on the application of
relativistic fluid dynamics (RFD) to heavy-ion collisions. (more…)